Apparatus for communication systems



R. H- BROWN APPARATUS FOR COMMUNICATION SYSTEMS July 30, 1946.

Filed Aug. 30, 1945 IN VENTOR. flobemEBmwzz.

Hrs Arron var.

Patented July 30, 1946 APPARATUS FOR COMMUNICATION SYSTEMS Robert H. Brown, Westview, Pa., assignor to The Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application August 30, 1945, Serial No. 613,627

Claims.

My invention relates to apparatus for communication systems, and more particularly to calling apparatus for telephone systems.

A single pair of line wires is often used as a transmitting channel common to several different communication systems. For example, a two wire line circuit along a railway may be used for a. dispatchers telephone circuit, a commercial telephone circuit, a telegraph circuit and a centralized traflic control circuit. In each such circuit, the current is of a characteristic individual to the system and in the centralized trafiic control system, coded current of a time code type may be used. Thus, an operator at an oifice governs through the centralized traffic control circuit the position of train signaling devices located at different stations along the railway, current pulses of a preselected code being used to select a desired station and a particular signaling device at the selected station. Also, in such centralized traffic control systems coded current is transmitted from the different stations to the cffice to indicate the positions of the signaling devices and the positions of trains in the vicinity of such stations.

In such a centralized traffic control systems telephone communication is usually provided between each of the stations and the office, so

that a train crew member or a maintainer may converse with the office operator in matters pertaining to train movements or the condition of the train signaling devices. In such telephone facilities it is advantageous to have apparatus wherewith a person at any one of the stations can call the oflice operator. Such calling apparatus cannot readily be of the common form for telephone calling apparatu because of the interference with the codes of the centralized traffic control system. Also, this calling means must be free from interference with the currents used by the other facilities of the line circuit. Again, the calling indicators effected by the calling means must be of a distinctive character because at such an ofilce there are usually several different indication bells and lights to indicate train movement and other operating conditions. Furthermore, this calling means must be of relatively high sensitivity to provide reliable operation through a relatively long line circuit and because of wide variations of the voltage of the usual power source. At the same time the calling means must be free from operation by conversation and noise energy that appears on the line circuit.

In View of the foregoing conditions. a feature of my invention is the provision of novel and improved calling apparatus for communication systems.

Another feature of my invention is the provision of improved voice operated calling apparatus for a telephone system.

Again, a feature of my invention is the provision of calling apparatus of the type here involved incorporating novel time delay means which may readily be adjusted as to its delay period, and which means requires that the calling current be continued during the full delay period and if there is an interruption in the calling current, the full delay period is reestablished and the timing cycle must be restarted.

Still another feature of my invention is the provision of calling apparatus of the type here contemplated incorporating novel mean to provide an answer back to the person calling.

Another feature of my invention is the provision of improved calling apparatus of high sensitivity and which is readily adjustable as to its sensitivity.

Other features, objects and advantages of my invention will appear as the specification progresses.

The foregoing objects, features and advantages of my invention are accomplished through an omce calling unit which incorporates a power pack, a detecting means, a time delay means and an answer back device.

The power pack is adaptable of converting energy from a single alternating current source into direct current and alternating current of voltage suitable for tne different elements of the apparatus.

The detecting means includes a receiving or selector circuit, a first electron tube and a first control relay. The selector circuit is coupled to the line circuit and is tuned to pass a preselected band of voice frequencies. The energy passed by this selector circuit is applied to a control electrode of the first electron tube which is preferably of the controlled ionization type and the tube i tripped in response to such energy and when thus made conductive the first control relay is energized by the conduction current. Thus this detecting means is responsive to energy of the calling current frequency regardless of the origin of such energy. A biasing voltage obtained from the power pack is applied to the first electron tube and it is made adjustable and the calling can be set for high sensitivity. Also the sensitivity of the calling unit can be adjusted in the 3 field to the condition of the particular line circuit with which it is connected.

The time delay means includes a capacitor, 2. second electron tub-e and a second control relay. The capacitor is connected to the power pack and is charged at a preselected direct voltage. This capacitor is also connected to the second tube to provide that tube with a bias voltage that prevents the tube from being normally tripped. When the first control relay is energized, this capacitor of the time delay means is disconnected from the power pack and is connected to a discharge circuit including a resistor and the capacitor is discharged at a predetermined rate. When the discharge of the capacitor is carried far enough the bias of the second tube is reduced to a point where that tube is tripped and the second control relay is energized by the conduction cunrent of the tube. If for any reason the discharging of the capacitor is interrupted it is quickly recharged and the time cycle must be restarted. This arrangement provides a selective control against random noise and conversation of the calling current frequency band operating the second relay, such noise energy and conversation energy of this frequency band being ordinarily of very short duration, so that the capacitor is not discharged for the required period to trip the second tube. The second electron tube is provided with a stick feature by which the tube is retained conductive regardless of the charge of the capacitor once the tube has been tripped.

The second control relay controls a call indicator, such as a lamp, and this visual indicator once established remains in effect until the operator acknowledges the call by operating a reset switch. The second relay also governs a novel circuit associated with the first tube and through which novel circuit the first tube is transferred from the selector circuit to the time delay means and the first tube is rendered conductive for a predetermined time interval. A call bell is interposed in the anode circuit of the first tube in place of the first control relay, and when the first tube is tripped for this time interval the call bell is rendered active as an audible call indication.

The second control relay when energized also completes an energizing circuit for the answer back device which is preferably of the buzzer type, and this buzzer is operated to supply to the line circuit an answer back current.

I shall describe one form of apparatus .embodying my invention, and shall then point out the novel features thereof in claims.

The accompanying drawing is a diagrammatic view showing a form of apparatus embodying my invention when used in a centralized traffic control system for railways.

It is to be understood that the invention is not limited to a centralized traffic control system in its use and this one use serves to illustrate the many places the apparatus is useful.

Referring to the drawing, the reference characters Y and Z designate line wires which extend along a railway, and which are used as a circuit for different facilities, such as a dispatchers telephone circuit, a telegraph circuit anda centralized traffic control circuit. It is to be understood that the invention is not limited to a pair of line wires used in several different circuits and the line wires Y and 2 may be used for telephone communication only. In order not to complicate the drawing, these facilities except that of the elephone communication associated with the centralized trafiic control system are not shown. The type of centralized traffic control system is immaterial and it may be any one of several of the well known arrangements, The office of such a system is indicated by a dot and dash rectangle OF and one station of the system is indicated by a dot and dash rectangle ST, only the one station being shown since the stations will have substantially duplicate facilities, and this one station is sufficient to illustrate the invention.

The station ST is provided with a telephone set which includes a transmitter 2 and a receiver 3 of the standard form and which set is connected across the line wires through a connection which preferably includes a blocking capacitor 4.

The office GE is provided with a telephone set TS, a source of alternating current, a calling unit indicated as a whole by a dotted rectangle CU, calling indicators comprising a lamp 5 and a bell 6, and a reset switch RS.

The office telephone set TS may be of any suitable form and may comprise a standard hand set or a loud speaker or a combination of these devices.

The office source of alternating current having terminals BX and CK may be of a usual commercial frequency and this source is connected to terminals of the calling unit CU, terminal BX of the source being connected to terminal "I of the calling unit and terminal CX of the source being connected to the two terminals 8 and 9 of the unit, the connection to terminal 9 including a normally closed contact iii of reset switch RS, as will be readily understood by an inspection of the drawing.

The calling unit CU includes a power pack comprising a transformer Tl, a twin diode rectifier tube RT, and two potentiometers, one including a resistor R3 and the other including two resistors R 3 and R5 in series; a detecting means comprising a transformer T2, a first electron tube Vi and a first control relay A; a time delay means comprising a capacitor Cl, a second electron tube V2 and a second control relay B; and an answer back device AB.

The power pack of the unit CU is energized from the office source of alternating current and provides filament voltage from the tubes and a direct voltage suitable for the biasing of the tubes. A primary Winding H of transformer TI is connected to terminals l and s, and a first secondary winding E2 of the transformer is connected to the filaments of the tubes as indicated conventionally in the drawing, a mid'terminal of secondary winding l2 being connected to ground electrode IS. A second secondary winding M of transformer Tl is connected across the anodes of the two sections of the twin diode tube RT, and the cathodes of these two sections of the tube are connected in multiple to one terminal of the potentiometer resistor R3 and to one terminal of the potentiometer resistors R4 and R5. The other terminal of resistor R3 and the other terminal of resistors R 8 and R5 are connected to a mid terminal of the secondary winding M. Consequently a direct voltage is created across these potentiometers due to the rectified current, the value of such direct Voltage being predetermined according to the proportioning of the parts. A by-pa-ss capacitor C2 is preferably connected across the potentiometer-s to by-pass the alterhating current ripples of the rectified current.

The detecting means is provided with a receiving or selector circuit which comprises the two parts of a primarywinding l5 of transformer T2 connected in series through'a capacitor I6 and which circuit is connected to terminals I1 and I8 of the unit CU and which terminals I! and I8 in turn are connected across the line wires Y and Z in multiple with the oflice telephone set. This selector circuit is tuned to pass a preselected band of voice frequencies which, for illustration, may be taken as being of the order of 1000 cycles per second, but other voice frequencies can be used. I have found in practicing the invention that a calling band produced by having the person calling to whistle or blow into the telephone transmitter is satisfactory.

A secondary winding I9 of transformer T2 is connected to a control electrode of tube VI to control this tube in response to the energy passed by the selector circuit. To be explicit, a circuit can be traced from the control electrode of tube VI, through resistor 2I, back contact 22 of control relay B, secondary winding I9, intermediate terminal 23 of the potentiometer resistor R3, a portion of resistor R3 and wire 24 to cathode 25 of the tube VI. It is clear that the control electrode 29 is provided with a bias voltage from the resistor R3, the control electrode being negative in potential with respect to the cathode by a voltage predetermined by the setting of the intermediate terminal 23. I preferably mechanically connect the intermediate terminal 23 to a manual operable knob, not shown, mounted on the calling unit for ready adjustment of the bias voltage for the tube VI This negative bias voltage for the tube VI is opposed by the positive half cycles of the electromotive force induced in the winding I9 due to the current passed by the selector circuit.

Tube VI may take any one of several types and is disclosed as a hot cathode gas filled tetrode. Tube VI is provided with an anode circuit receiving energy from the alternating current source, the terminal BX of the source being connected through terminal I of the unit to a winding of the control relay A and thence through resistor 25, back contact 21 of relay B, anode 28 and tube space to cathode 2-5 of tube VI, wires 24 and 29,

terminal 9 of the unit CU, and contact II) of the reset switch RS to the other terminal CX of the alternating current source. The parts are so proportioned that the bias negative voltage applied to control electrode 20 from resistor R3 prevents the anode voltage from tripping tube VI but a relatively small voltage of secondary winding I9 to oppose the bias voltage causes the tube to be tripped and relay A to be energized by the cur rent passed by the tube. A capacitor 30 is connected across the winding of relay A to retain the relay energized during the negative half cycles of the alternating current when the tube is nonconductive.

Capacitor CI of the time delay means is connected across resistor R4 through a charging circuit which can be traced from the top terminal of resistor R4 as viewed in the drawing, through capacitor CI, back contact 3| of relay B and back contact 32 of relay A to the junction terminal of resistors R4 and R5, and the capacitor CI is charged at a direct voltage equal to the voltage drop across resistor R4, When relay A is picked up in response to the tripping of the electron tube V I of the detecting means, the condenser CI is disconnected from the charging circuit and is connected to a discharging circuit and which discharging circuit can be traced from the top terminal of capacitor CI through a portion of resistor R6, an adjustable intermediate terminal 33, front contact 34 of relay A and back contact 3| of relay B to the other terminal of the capacitor. Thus capacitor CI is discharged at a rate predetermined by the value of the resistor R6 and the setting of the intermediate terminal 33. The intermediate terminal 31:! is preferably mechanically connected to a manually operable knob, not shown, mounted on the calling unit for adjustment of the discharge rate of the capacitor CI.

The electron tube V2 of the time delay means may be any one of several well-known types and is shown as a hot cathode gas filled tetrode. Tube V2 is provided with a connection to the capacitor CI to bias the tube according to the charge of the capacitor. Normally, the positive plate of capacitor CI is connected to cathode 35 of tube V2 through wire 40 and the negative plate of the capacitor is connected to control electrode 36 of the tube through back contact 31 of relay B and resistor 38. Thus control electrode 35 is negative in potential with respect to the oathode 35 by a voltage predetermined by the charge of capacitor CI. Tube V2 is provided with an anode circuit receiving energy from the alternating current source, the circuit extending from terminal BX of the source through terminal I of the unit CU to a windin of relay B, anode 39 and tube space to cathode 35 of tube V2, Wires 40, 24, 29 to terminal 9 of unit CU and terminal II! of reset switch RS to terminal CX of the alternating current source.

The parts are proportioned for the voltage applied to the anode of tube V2 to be inefiective to trip the tube when the control electrode 36 is biased by the voltage of capacitor CI, but when capacitor CI is discharged to a preselected point the tube V2 is tripped and relay B is energized. Relay B when energized to close front contact 4I completes a stick circuit connection between control electrode 36 and cathode 35 and the tube retained conductive due to control electrode 36 and cathode 35 being at substantially the same potential. That is to say, the tube V2 is retained conductive until such time as the anode circuit is opened due to the operation of the reset switch RS in a manner to appear later.

With relay B picked up to close front contact 42 the call indicator 5 is energized by current flowing from the top terminal of the secondary winding I2 of transformer TI through wire 45, front contact 42 of relay B, terminal 46 of the unit CU, lamp 5, and ground electrodes 41 and I3 to the mid terminal of secondary winding I2. That is to say, the call indicator lamp 5 is illuminated as long as the control relay B is energized.

Relay B on picking up to close front contacts 43 and 48 completes circuits by which tube VI is disconnected from the receiving or selector circuit and is connected to the'capacitor CI andby which the relay A is disconnected and bell 6 is interposed in the anode circuit of tube VI. Since relay B is not energized until the capacitor CI is substantially discharged, the bias voltage applied from capacitor CI to control electrode 20 of tube VI through the connection including front contact 48 of relay B is low and tube VI is tripped and anode current flows through the bell 6 from terminal BX of the current source to terminal 49 of unit CU and thence through resistor 50, front contact 43 of relay B, anode 28 and tube space to cathode 25, wires 24 and 29, terminal 9 and contact IQ of switch RS to terminal OK. The bell 6 is thus sounded as an audible call indicator while tube VI is retained conductive by this circuit. Relay A when deenergized to close back contact 32 completes the charging circuit for capacitor CI, the charge being built up on the capacitor at a predetermined rate because resistor R2 is now included in the charging circuit due to relay B picking up to open back contact 3|. After a preselected time interval of say 2 seconds, the charge on capacitor CI builds up to a voltage sufiicient to prevent tube VI from tripping during the positive half cycles of the alternating current and tube VI is deionized and thebellfi stops ringing.

The picking up of relay B to close front contact 44 completes a circuit from the answer back device AB, which device is energized and operated to supply a current to the line circuit.

The device AB may be of any one of several different forms, and is shown as a buzzer energized by alternating current from secondary winding 52 of transformer TI, the circuit extending from the top terminal of secondary wind- 1 ing I2, through wire 45, front contact 44 of relay B, secondary winding ii of transformer T2, winding of the buzzer AB, back contact 52 of the buzzer, and ground electrodes 53 and, I3 to the mid terminal of secondary winding I2. With the buzzer thus energized, current is transferred from winding 5I of transformer T2 to winding I5 and from winding I5 current is applied to the line wires for operating the telephone receiver at the station ST. The parts are so proportioned that this answer back current thus applied to the line circuit would produce a distinctive tone in the telephone receiver at the station ST.

In describing the operation of the apparatus I shall assume that power is applied to the unit CU and no energy is received from the line circuit. Under this normal condition, the tubes VI and V2 are biased to a non-conductive condition by voltages supplied from the power pack and relays A and B are deenergized so that the call indicators 5 and I3 are inactive.

Assuming that a person at station ST wishes to telephone to the ofilce operator, the telephone set at the station is removed from the hook and the calling person either whistles or blows, as prearranged, into the transmitter at the station. This action creates a calling current of a preselected frequency band, and which calling current is applied to terminals I? and I8 of the ofiice calling unit and is passed by the selector circuit.

The resultant electromotive force induced in secondary winding I9 is supplied to the control electrode 257 of tube VI to drive the control electrode in the positive direction with the result that the tube is tripped causing the control relay A to be energized. Relay A on picking up transfers capacitor CI of the time delay means to its dis charge circuit and the direct voltage across the capacitor is discharged at a rate predetermined by the setting of the intermediate terminal 33 of the resistor R6 as described h'ereinbefore.

Assuming the person calling continues his call for a preselected interval, the discharge of capacitor CI is carried to a point where the bias of tube V2 is reduced in voltage to a point that permits the tube V2 to trip and the control relay B is energized. With control relay 3 energized to close front contact 4|, tube V2 is sealed in by its stick feature and remains conductive regardless of the charge on the capacitor Cl. The calling lamp 5 is now illuminated due to the closing of front contact 42 of relay B and becomes a calling indication. Relay B on picking up to close front contact 43 and 48 brings the transfer circuits associated with tube VI into play and 8 the call hell 6 rings for a preselected period of say two seconds.

The picking up of control relay B also energizes the answer back device AB and the device AB is operated to suppl current to the line circuit to produce a tone in the telephone receiver at station ST, so that the person calling knows that his call has been properly received at the office.

The office operator in acknowledging this call will take his telephone set to initiate communication with the calling person at the station ST. The operator also will operate the reset switch RS which opens the anode circuit for tubes VI and V2 and the calling unit is restored to its normal condition ready to receive another call.

Assuming that energy of the calling current frequency appears in the line circuit due to some noise condition or due to a particular syllable or word in the conversation, such energy is passed by the selector circuit, the tube VI is tripped, and the control relay A is picked up. This noise energy will ordinarily be of short duration and consequently the tube VI is deionized and relay A released before the capacitor CI is discharged to a point where the tube V2 is tripped. With relay A thus released, capacitor CI is reconnected to its charging circuit and the charge on the capacitor is immediately restored and the full delay period of the device becomes reestablished. Thus, a call will not be made effective due to a succession of pulses of noise energy occurring in short intervals apart.

Apparatus here disclosed has the advantages that it is adjustable for relatively high sensitivity and a call can be accomplished through a relativel long line circuit. A relatively large variation in the voltage of the source of power does not greatly Vary the sensitivity of the unit or the established delay period. The sensitivity and the delay period of the device can both be quickly adjusted to suit the operating conditions of the line circuit with which the apparatus is associated. Successive pulses of current of the calling frequency due to noise energy or due to the conversation will not ordinarily establish a call because of the delay period of the device.

Although I have herein shown and described but one form of apparatus for communication systems embodying my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In apparatus for communication systems, the combination comprising, supply means to transmit a calling current, a selector circuit adapted to receive such calling current, a controlled ionization tube provided with an anode cathode circuit including a power source and an electromagnet and with a control electrode cathode circuit including a capacitor, a charging circuit including a source of direct voltage, a discharging circuit including a resistor, circuit means governed by said selector circuit and operable to connect either said charging circuit or said discharging circuit across said capacitor according as calling current is not or is being received, said charging circuit to charge the capacitor at a voltage as required to bias said control electrode negative in potential with respect to said cathode to retain the tube deionized, said discharging circuit to discharge said capacitor at a preselected rate to permit said tube to be ionized and said electromagnet energized in a preselected time interval, and a call indicator governed by said electromagnet.

2. In apparatus for communication systems, the combination comprising, means operable at times to transmit a calling current of a given characteristic, a receiving circuit adapted to receive such calling current, a controlled ionization tube provided with an anode cathode circuit including a power source and a relay and with a control electrode cathode circuit including a capacitor, a charging circuit including a source of direct voltage, a discharging circuit including impedance, a circuit element operatively associated with said receiving circuit to connect either said charging circuit or said discharging circuit to said capacitor according as said calling current is not or is being received, said charging circuit to charge said capacitor to bias said tube as required to retain the tube non-conductive, said discharging circuit to discharge the capacitor to reduce the bias of the tube for the tube to be tripped and said relay energized in a predetermined time interval, and a call indication circuit including a front contact of said relay.

3. In apparatus for communication systems, the combination comprising, means to supply at times a calling current of a given frequency band, a selector circuit tuned to receive current of said frequency band, a controlled ionization tube, said tube having an anode cathode circuit including a power source and a relay, a control electrode cathode circuit including a capacitor for said tube, a charging circuit including a direct Voltage source having connections to said capacitor, a discharging circuit including a resistor having connections to said capacitor, said charging circuit eiiective to charge the capacitor as required to bias the tube to a non-conductive condition, said discharging circuit efiective to discharge the capacitor as required to trip the tube and energize said relay in a preselected time interval, means governed by said selector circuit to close either the connections of the charging circuit or of the discharging circuit according as calling current is not or is being received whereby said relay is energized only when the calling current cont nues for said preselected time interval, and

a call indication means governed by said relay and made active only when the relay is energized.

4. In apparatus for communication systems, the combination comprising, means to supply at times a calling current of a given frequency band, a receiving circuit tuned to rece ve current of said frequency band, a controlled ionization tube, said tube having an anode cathode circuit including a power source and a relay, a control electrode cathode circuit including a capacitor for said tube, a charging circuit including a direct voltage source having connections to said capacitor, a discharging circuit including a resistor having connections to said capacitor, said charging circuit effective to charge the capacitor as required to bias the tube to a non-conductive condition, said discharging circuit effective to discharge the capacitor as required to trip the tube and energize said relay in a preselected time interval, means governed by said receiving circuit to clos either the connections of the charging circuit or of the discharging circuit according as calling current is not or is being received, means including a front contact of said relay to connect said control electrode to said cathode to retain the tube conductive, means including a front contact 10 of said relay to indicate the reception of such calling current,'and a manually operable reset switch to open said anode cathode circuit to deionize the tube.

'5. In apparatus for communication systems, the combination comprising, an oiiice and a station connected by a line circuit, station telephone means to supply at times to said line circuit a calling current having a given voice frequency band, an ofiice receiving circuit connected to said line circuit and tuned to pass said frequency band, an oflice gas filled tube provided with an anode circuit including a power source and a relay and with a control electrode circuit including a capacitor, ofiice means operatively associated with said receiving circuit to connect a source of direct voltage or a resistor across said capacitor according as said callin current is not or is received, said direct voltage source to charge said capacitor to bias the tube as required to retain the tube deionized, said resistor to discharge the capacitor as required to trip the tube and energize said relay in a predetermined time interval, an indication circuit including a front contact of said relay and an indicator to indicate reception of the calling current, an office vibrator coupled to said line circuit, and an operating circuit including a front contact of said relay to energize said vibrator to supply an answer back current to said station telephone means.

6. In apparatus for communication systems, the combination comprising, calling means operable to supply at times a calling current, a selector circuit to receive such calling current, a first controlled ionization tube provided with an anode circuit including a power source and a first relay and with a control electrode circuit including an element coupled to said selector circuit, said anode and control electrode circuits to ionize said first tube and energize said first relay in response to reception of said calling current, a second controlled ionization tube provided with an anode circuit including a power source and a second relay and with a control electrode circuit including a biasing means, said biasing means effective to retain said second tube deionized, means including a front contact of said first relay connected to said biasing means to vary its effect at a preselected rate to ionize said second tube and energize said second relay in a predetermined time interval whereby said second relay is energized only when the calling current continues uninterrupted for said time interval, and a call ind cator governed by said second relay.

7. In apparatus for communication systems, the combination comprising, transmitting means to transmit from a station to an office a calling current of a preselected voice frequency band, a selector circuit at said oflice tuned to pass said frequency band and arranged to receive said calling current, a first c-fiice gas tube having an anode circuit including a power source and a first relay and a control electrode circuit including a winding coupled to said selector circuit, said first gas tube normall non-conductive and tripped in response to said calling current to energize said first relay, a second ofiice gas tube having an anode circuit including a power source and a second relay and a control electrode circuit including a capacitor, means including a back contact of said first relay to charge said capacitor at a direct voltage and said capacitor poled to render said second tube control electrode negative in potential with respect to the tube cathode to retain the tube non-conductive, means including a 11 front contact of said first relay to connect a resistor across said capacitor to discharg the ca pacitor to a voltage where, said second tube is tripped in a predetermined time interval to energize said second relay, and a call indication circuit including a front contact of said second relay.

8. In apparatus for communication systems, the combination comprising, transmitting means to transmit from a station to an oifice a calling current of a preselected voice frequency band, a receiving circuit at said ofiice tuned to pass said frequency band and arranged to receive said calling current, a first o-ffice gas tube having an anode circuit including a power source and a first relay and a control electrode circuit including a winding coupled to said receiving circuit, said first gas tube normally non-conductive and tripped in response to said calling current, to energize said first relay, a second office gas tube having an anode circuit including a power source and a second relay and a control electrode circuit including a capacitor, means including a back contact of said first relay to charge said capacitor at a. direct voltage and. said capacitor poled to render said second tube control electrode negative in potential with respect to the tube cathode to retain the tube non-conductive, means including a front contact of said first relay to connect a resistor across said capacitor to discharge the capacitor to a voltage where said second tube is tripped in a predetermined time interval to energize said second relay, means including a front contact of said second relay to connect said second tube control electrode to the cathode. of the tube to retain the tube conductive, and a call indicator lamp controlled through, a front contact of said second relay.

9. In apparatus for communication. systems, the combination comprising, transmitting means to transmit from a station to an ofiice a calling current of a preselected voice frequency band, a receiving circuit at said office tuned to pass said frequency band and arranged to receive. said calling current, a'first office gas tube having an anode circuit including a power source and a first relay and a control electrode. circuit including a Winding coupled to said receiving circuit, said first gas tube normally non-conductive and tripped in response to said calling current to, energize said first relay, a second ofiice gas tube having an, anode circuit including a power source and a second relay and a control electrode circuit including a capacitor, a charging circuit including a back contact of said first rela and a direct voltage source to charge said capacitor as required to retain said second tube non-conductive, a discharging circuit including a front, contact of said first relay and a resistor connected to said capacitor to discharge said capacitor at a rate as required to trip said second tube and energize said. second relay in a predetermined time interval, a call. bell, means including contacts of said second relay to disconnect said first relay and connect the call bell, in the anode circuit of said first tube, means including a contact of said second relay to interpose a resistor in said charging circuit to delay the charging of the capacitor for a preselected time period when the second relay is energized, and means including contacts of said second relay to switch said capacitor to the control electrode circuit of the first tube to trip thev first tube and operate said call bell for said preselected time period, when said second relay is energized.

10. In combination, a first and a second. gas tube, a first and a second relay, a call bell, said first tube provided with an anode circuit having two paths one of which paths includes a current source, said first relay and a back contact of said second relay; and the other of Which paths includes said current source, said call bell and a front contact of said second relay; said second tube provided with an anode circuit including a current source and said second relay, said first tube provided with a control electrode circuit having two paths one of which paths includes a back contact of said second relay, a winding and a bias voltage source; and the other of which paths includes a front contact of said second relay and a capacitor; said second tube provided with a control electrode circuit including said capacitor, a charging circuit including a back contact. of said first relay and a voltage source connected to said capacitor to charge the capacitor to create a bias voltage for said second tube to retain the tube non-conductive, a discharging circuit including a front contact of said first relay and a resistor connected to said capacitor to discharge the capacitor at a given rate to trip said second tube in a predetermined time interval, means including a contact of said second relay to interpose a resistor in said charging circuit when the second relay is energized to charge the capacitor in a preselected time period, and supply means to at times induce in said winding an electromotive force to trip said first tube whereby said call bell i sounded for said preselected time period when said electromotive force continues for said predetermined time interval.

ROBERT. E. BROWN. 

